Increasing demands on fossil fuel energy reserves, ever increasing energy costs and global warming have led to a heightened interest in alternative fuels. In the near future alternative carbon neutral renewable liquid fuels are required to totally displace decreasing reserves of petroleum-derived transport fuels that contribute to global warming. Biodiesel from oil crops and bioethanol from sugarcane and corn are being produced in increasing amounts as renewable biofuels, but their production in large quantities is not sustainable. To satisfy the biodiesel energy need in the United States (0.53 billion m3), 111 million hectares or 61% of available cropping land would have to be cultivated with oil palm, which is clearly an unrealistic option. Alternative sources of biological starting materials or energy feedstocks are required and may potentially be offered by marine macro/micro-algae. To produce an equivalent amount of biodiesel from microalgal feedstocks would require only 3% of the cropping area of the United States, a much more feasible option and thus the importance of microalgae as an alternative feedstock for biodiesel production becomes obvious. Approximately half of the dry weight of microalgal biomass is carbon-rich and derived from carbon dioxide; production of 100 tonnes of biomass will fix 183 tonnes (approx.) of carbon dioxide. This may come from existing fossil fuel power plants at little or no cost. Ideally microalgal biodiesel could be carbon neutral with the energy required for processing the algae coming from methane produced by anaerobic digestion of biomass left over after the oil has been extracted.
Another important advantage of microalgae is that, unlike oil crops, they grow extremely rapidly and commonly double their biomass within 24 h. In fact, the biomass doubling time for microalgae during exponential growth can be as short as 3.5 h.
Microalgae comprise a vast group of photosynthetic, heterotrophic organisms, which have an extraordinary potential for cultivation as energy crops, converting sunlight, water and carbon dioxide to algal biomass. Microalgae encompass an immense range of genetic diversity and can exist as unicells, colonies and extended filaments. It has been estimated that between 200,000 and several million species of microalgae may exist, compared with about 250,000 species of higher plants. They are ubiquitously distributed throughout the biosphere and grow under the widest possible variety of conditions. Growth media are generally inexpensive, sea water supplemented with commercial nitrate and phosphate fertilizers, and a few other micronutrients can be used for growing marine microalgae. Fresh and brackish water from lakes and rivers can be used for freshwater species. Economics dictates the biomass must be produced at minimal expense, using freely available sunlight and is thereby affected by fluctuations such as daily and seasonal variations in light levels. Microalgae can be grown on a large scale in photo bioreactors.
While the full extent of the global resource of macroalgae is not yet fully known, these significantly larger algae include an equally diverse range of species. More than 60% of the biomass weight of brown, green and red macroalgae (also known as seaweeds) can be comprised of carbohydrate. Specific phycocolloids derived from these macroalgae (e.g. alginates, carageenans, sulphated galactans, agars) have a long history of use in food and pharma applications. These and other carbohydrates present in macroalgae can be converted to fermentable sugars to provide an additional (not sole) feedstock source for biofuel production. However, due to the extensive repertoire of additional high-value molecules (e.g. pigments, polyphenols, protein, oils, minerals, vitamins and trace elements) that are present in the brown, green and red macroalgae, they represent a rich resource of high-value bioproducts.